Hafnium doped cap and free layer for MRAM device

ABSTRACT

A high performance MTJ, and a process for manufacturing it, are described. A capping layer of NiFeHf is used to getter oxygen out of the free layer, thereby increasing the sharpness of the free layer-tunneling layer interface. The free layer comprises two NiFe layers whose magnetostriction constants are of opposite sign, thereby largely canceling one another.

FIELD OF THE INVENTION

The invention relates to the general field of MTJ devices withparticular reference to formation of the capping layer.

BACKGROUND OF THE INVENTION

Key to the performance of MTJs (magnetic tunnel junctions) for MRAM(magnetic random access memory) and read heads are: (a) well-controlledmagnetization of the pinned layer, large pinning field and high thermalstability, (b) integrity of the tunnel barrier layer, and (c)well-controlled magnetization and switching of the free layer.

For (a), the pinned layer of the MTJ device is typically a SyAF(synthetic antiferromagnetic) layer (e.g. AFM/CoFe/Ru/CoFe). Use of SyAFpinned layer in the MTJ structure not only improves thermal stabilitybut also minimizes the interlayer coupling field (offset field) betweenthe pinned layer and the free layer. For (b), the tunnel barriercommonly used is either a thin layer of amorphous AlOx or crystallineMgO. It has been shown, in the case of a NiFe free layer-MTJ made withamorphous AlOx and crystalline MgO barrier layers is capable ofdelivering dR/R (magnetoresistive change) more than 40% and 85%,respectively (1, 2). For (c), the free layer for the MRAM-MTJ is bestmade of a thin permalloy (NiFe) film for its reproducible and reliableswitching characteristics (i.e. low switching field (Hc) and switchingfield uniformity σHc). It is important that the MRAM-MTJ free layerexhibit low magnetostriction λ_(s) (lambda <1×10⁻⁶).

The typical cap layer for a conventional MTJ stack is a nonmagneticconductive metal such as Ta or TaN. The disadvantage of using a Tacapping layer is that, during thermal annealing, Ta diffuses into theNiFe free layer and not only reduces the free layer moment (Bs) but alsomakes the NiFe free layer very magnetostrictive with a λs>5×10⁻⁶ (1). InMRAM application, we found that a high percentage of free layerswitching, in Ta capped NiFe-MTJs, is through a vortex structure whichresults in poor switching field uniformity. To eliminate NiFe/Tainter-diffusion, the prior art has used Ru to cap the NiFe (free)-MTJ.However, dR/R of the Ru cap MTJ is severely degraded (from 40% to 30%).To reduce NiFe/Ta inter-diffusion while still preserving high dR/R, theNiFe (free)-MTJ can be capped with a Ru/Ta/Ru structure (1). Duringthermal annealing to fix the pinned layer magnetization direction, theintermediate Ta layer in the tri-layer cap is capable of getteringoxygen atoms in the underlying NiFe free layer. Consequently, the NiFefree layer is less oxygen contaminated and a sharper AlOx/NiFe interfaceis formed, resulting in an improved dR/R.

A routine search of the prior art was performed with the followingreferences of interest being found:

U.S. Patent Applications 2006/0114716 and 2004/0085681 (Kai et al) teacha composite free layer comprising two layers of NiFe with a layer of Hfin between. U.S. Pat. No. 7,072,208 (Min et al—Headway) shows a freelayer of NiFe with a dopant concentration of 1-40% by weight of Hf. U.S.Patent Application 2004/0257719 (Ohba et al) discloses a NiFe—Hf freelayer.

U.S. Pat. No. 7,067,331 (Slaughter et al) describes a free layer ofCoFeHf. U.S. Patent Application 2006/0119990 (Nishiyama et al) shows aNiFe free layer with Hf aggregated into the crystal grain boundary. U.S.Pat. No. 6,710,987 (Sun et al) discloses NiFe as a free layer alloyedwith Cr, Ta, Mo, Nb, or Zr to have low magnetization.

U.S. Pat. No. 7,026,673 (Abraham) shows NiFe free layer alloyed with Ge,B, V, Mb, or Os. This patent teaches the use of “low magnetizationmaterials” for high performance magnetic memory devices. Essentially, anMTJ is made with a thicker free layer of low magnetization materials toimprove AQF (array quality factor, defined as Hc/σHc). For a permalloy(NiFe19%) Ms=800 emu/cc, the low magnetization material is preferably aNiFe alloy having Ms less than 600 emu/cc. The low magnetizationmaterial comprises a Ni—Fe alloy, including one or more moment reducingelements such as Ge, B, V, Mo, or Os and combinations. Among theseelements, only V has an oxidation potential higher than Fe and Ni.

U.S. Pat. No. 6,903,909 (Sharma et al) teaches a NiFe pinned layer towhich an amorphizing agent Hf is added. U.S. Patent Application2006/0056114 (Fukumoto et al) describes a magnetic layer that can beNiFeHf. U.S. Patent Application 2002/0054462 (Sun et al) shows a freelayer of NiFe/CoFe with a barrier layer thereover comprising an alloy ofNi and a Hf element.

Additional references of interest included:

-   1. C. Horng et. al. 2003 Invention disclosure “A novel capping layer    for forming high performance MTJ for MRAM application”.-   2. R W. Dave et. al., “MgO-based tunnel junction material for    high-speed toggle MRAM”, Abstract ED-05, 2005 MMM conference.-   3. M. Nagamine et. al. “Conceptual material design for MTJ cap layer    for high MR ratio”, abstract ED-10, 50th MMM conference.-   4. C. Horng et. al. EMG06-005 “A novel method to form    nonmagnetic-NiFeMg cap for the NiFe(free layer)-MTJ to enhance    dR/R”.-   5. C. Homg & R. Tong, EMG06-011 “A novel material to cap the    NiFe(free layer)-MTJ to enhance dR/R and method of forming the cap    structure”, and I-IMG06-16 “A novel material to cap the NiFe(free    layer)-MgO-MTJ to enhance dR/R and method of forming the cap    structure”.-   6. M. Chen et. al. “Ternary NiFeX as soft biasing film in a    magnetoresistive sensor”, J. Appl. Phys, 69. n 5631-33 (1991).-   7. U.S. Pat. No. 7,026,673 “Low magnetization materials for high    performance magnetic memory devices”.

SUMMARY OF THE INVENTION

It has been an object of at least one embodiment of the presentinvention to improve the dR/R ratio of a MTJ device Another object of atleast one embodiment of the present invention has been to render thefree layer of said MTJ device free of oxygen contaminants.

Still another object of at least one embodiment of the present inventionhas been to increase the sharpness of the interface between the tunnelbarrier and free layers.

A further object of at least one embodiment of the present invention hasbeen to provide an improved capping structure for said device.

These objects have been achieved by using an MTJ cap layer that is alow-moment NiFeHf/Ta/Ru tri-layer. This layer is a powerful getter foroxygen trapped in the free layer. The relative electrode potentials(electro-negativity) are Hf<Mg<Nb<Zr<Ta . . . <Fe<Ni so a NiFeHf cap isthe most powerful for gettering oxygen contaminants from the underlyingNiFe free layer.

The NiFeHf cap is formed by co-sputtering NiFe and Hf. Co-sputteredNiFeHf when deposited on silicon oxide is nonmagnetic, but on NiFe it isweakly magnetic. The intrinsic dR/R (i.e. measured at zero biased field)for a NiFeHf cap MTJ is much higher than for a Ru cap MTJ. dR/R measuredat 300 mV biased for the NiFeHf cap MTJ has a 30% improvement over thatof the standard cap MTJ. V₅₀ of the NiFeHf cap MTJ is improved from 650mV (for Ru cap MTJ) to 750 mV. Also, the error count (EC) for the MRAMarray has been drastically reduced

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the invention for the case of a simple (single) freelayer.

FIG. 2 illustrates the case of a compound (bilayer) free layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, a capping layer containing tantalum has been shownby the prior art to provide a certain amount of gettering of oxygen inthe free layer. Gettering could, however, be even more effective if theoxygen-extracting layer were to be in direct contact to the free layer.This, however, introduces the possibility of alloying between the twolayers.

To solve this alloying problem, it is desirable to use a nonmagneticNiFeX cap (3) that includes a strong oxygen gettering agent X such asTa. In this way diffusion of element X into the NiFe free layer isgreatly reduced. To getter oxygen originating in the NiFe free layer,the X element in the nonmagnetic NiFeX cap should preferably have anoxidation potential higher than Ni and Fe. Thermodynamically, theelectrode potential (electronegativity) is Hf<Mg<Nb<Zr<Ta, <V<Fe<Co<Ni.

In the prior art (1, 3), it has been shown that the MTJ cap layer alsoplays a critical role in enhancing MRAM performance. We thereforeexperimented with using various nonmagnetic conductive materials, suchas TaN, NiFeMg (4), NiFeZr(5) and NiFeHf (5), to cap the MTJ. Amongthese cap materials, a NiFeHf cap was found to produce the mostsignificant improvement in dR/R. Accordingly an MTJ manufacturingprocess that incorporates this material has been engineered and will nowbe disclosed:

Referring now to FIG. 1, the process of the present invention beginswith the provision of suitable substrate 45 that was previously laiddown as part of the overall process. Seed layer 46 (which is not part ofthe invention) is commonly laid down (on this substrate) prior to thedeposition thereon of magnetic pinning layer 47.

Magnetically pinned layer (or layers) 48 is then deposited onto layer47, following which tunneling barrier layer 49 (typically alumina ormagnesia) is deposited on layer 48.

Next, free layer 50 is deposited onto tunneling barrier layer 49. Layer50 may be a single layer of NiFe, as shown in FIG. 1, or it may be acomposite of two NiFe layers, 50 a and 50 b, as shown in FIG. 2. In thelatter case, layers 50 a and 50 b are given compositions and thicknessesthat result in them having magnetostriction constants of opposite signso that, when combined they have a very low net magnetostrictionconstant (typically less than about 1×10⁻⁶).

Next, as a key feature of the invention, a first capping layer 51 ofNiFeHf, comprising about 15 atomic % Hf, is deposited onto the freelayer (or layers). A second capping layer 52 (of Ru on Ta) is depositedonto first capping layer 51.

The structure is then heated for a time and at a temperature that aresufficient for oxygen trapped in the free layer (or layers) to begettered by the NiFeHf layer 51 i,e, to diffuse from the free layer intothe NiFeHf layer. At the same time some hafnium will diffuse into thefree layer(s), as will be discussed in greater detail below. The netresult is a sharpening of the interface between the tunnel barrier layerand the free layer(s) which results in a significant improvement in thedR/R performance of the device.

The time and temperature for the above described heating process havetypically been for between about 2 and 10 hours at a temperature betweenabout 250 and 280° C.

Experimental: Results and Discussion

The sputter system used was Anelva C-7100-Ex Thin Film Sputtering System(4,5). Nonmagnetic-NiFeHf cap layers were made using a Hf and NiFecosputtering method. Nonmagnetic-NiFeHf herein refers to thick NiFeHffilms deposited on a Si0₂/Si substrate that show no magnetic moment. ANiFe(21%) or NiFe(12%) target was used to co-sputter with the Hf targetto form [NiFe(21%)]xHf(1−x) or [NiFe(12%)y]Hf(1−y) cap, respectively. Inthe former, nonmagnetic NiFeHf is formed by co-sputtering NiFe(21%)/Hfusing 400 W/200 W power, while the latter was by co-sputteringNiFe(12%)/Hf using 400 W/120 W power.

The composition of the deposited NiFeHf alloy films was analyzed bymeans of TEM (transmission electron microscopy). Thenonmagnetic-NiFe(21%)-Hf alloy was found Ni(56.8 at %)-Fe(15.2 at%)-Hf(28 at %). The nonmagnetic-NiFe(12%)-Hf wasNi(75%)-Fe(10%)-Hf(15%). In the following, the Ni(56.8%)Fe(15.2%-Hf(28%) cap will be referred to as NiFeHf(28%) andNi(75%)-Fe(10%)-Hf(15%) cap as NiFeHf(15%).

MTJs with the following MTJ stack configuration were made:

Bottom conductor/Buffer layer/Pinned ferromagnetic layer/AlOX/NiFe(21%)33(free)/Ni FeHf(28)/Ta30/Ru 100(cap)

The AlOx tunnel barrier layer was formed by ROX (radical oxidation) of8.25 A-thick Al. For reference, MTJ stack capped POR Ru30/Ta30/Ru100tri-layer was also made. The deposited MTJ stacks were further processedto have a 280° C.-5 hrs-10 kOe annealing. Magnetic performanceproperties of the MTJs were measured using CIPT, B-H looper andmagnetostriction (lambda) tester. The results are shown in TABLE I:

TABLE I RA, Ω- dR/R Row Free layer Capping layer Bs H_(c) μm² dR/R gainlambda 1 NiFe(21%)33 Ru30/Ta30/Ru100 0.614 4.55 946 40.8% Ref 1.1E−6 2NiFe(21%)33 NiFeHf 28% 25/Ta30/Ru100 0.760 5.91 953 54.9% 35% 7.1E−6

As listed in rows 1 and 2 of TABLE I, Bs=0.614 nw (nanoweber) for a 8″wafer was measured for the reference MTJ having a NiFe33 free layer,while Bs=0.76 nw is measured for the NiFeHf(28%) cap NiFe33-MTJ. This is0.15 nw larger than that of the Ru cap MTJ. This indicates that the 400W/200 W co-sputtered NiFeHf cap on top of the NiFe free layer ismagnetic (as will be confirmed later). Moment (Ms) of 0.15 nw for a 8″wafer amounted to a 8A-thick NiFe(21%) layer. Since the lattice matchbetween the NiFe/NiFeHf layers is better than that between the NiFe/Rulayers, the MTJ moment increase may also come from “reactivation” of aNiFe “dead layer” when Ru is used to cap the NiFe free layer.

As shown in Table I, dR/R of the NiFeHf(28%) cap MTJ is around 55%,while the Ru cap MTJ is 40.8%. The enhancement is 35%, which is asignificant improvement. We had implemented this NiFeHf(28%) cap-MTJ aspart of a 1 Mbit MRAM chip. As shown in Table I, Hc measured on theNiFeHf cap and Ru-cap MTJ full film stack is, respectively, 5.91 Oe and4.55 Oe; switching field (Hsw) for the patterned 0.3×0.6 μm² bit in the1-Mbit MRAM array was measured to be around 100 Oe while the switchingfield for the reference patterned bit was 37 Oe.

In the 1-Mbit circuit, the maximum write current of 10 mA is not able toswitch the NiFeHf-MTJ patterned bit. In the patterned devices, this highswitching field is related to the thicker free layer (i.e. Bs). It isnoted that high magnetostriction (lambda) is also responsible for thehigh switching field. In this respect, high magnetostriction of theNiFeHf (28%) cap MTJ may indicate that Hf in the NiFeHf(28%) cap isbeing diffused, during annealing, into the underlying NiFe(21%) freelayer (6). It is noted V₅₀ of the NiFeHf(28%) cap MTJ is measured to bearound 750 mV, while V₅₀ for the reference Ru cap MTJ is around 660 mV.

To solve the magnetostriction problem, a nonmagnetic-NiFeHf cap layerwas made by co-sputtering NiFe(12%) and Hf. The composition of this newcap was Ni(75%)-Fe(10%)-Hf(15%) and is designated as NiFeHf(15%). To becompatible with the NiFeHf(15%) cap, the MTJ free layer was changed to acomposite free layer made of NiFe(21%)t1/NiFe(12%) t2 or NiFe(17.5%)t3/NiFe(12%)t4. It is noted that magnetostriction of the NiFe(21%) ispositive, while that of NiFe(17.5%) is negative and that of NiFe(12%) iseven more negative. Thus, by adjusting the free layer thicknesses t1 andt2 or (t3 and t4), MTJ magnetostriction could be tuned to a very lowvalue (i.e. <1×10-6). Magnetic performance properties for composite NiFe(free)-MTJ capped with NiFeHf(15%) are illustrated in TABLE II.

TABLE II Magnetic performance properties of the compositeNiFe(21%)NiFe(12%) free MTJ stack NiFe free cap layer annealing Bs HcRA, Ωμm² dR/R lambda NiFe(21%)8/NiFe(12%)22 NiFeHf(15%)25/Ta30/Ru100280° C. 0.64 4.86 1060 46.2 −9.0E−8 NiFe(21%)10/NiFe(12%)23NiFeHf(15%)25/Ta30/Ru100 ″ 0.68 4.78 1080 44.5 1.1E−6 NiFe(21%)33Ru30/Ta30/Ru100 ″ 0.61 4.55 852 40.0 1.1E−7

As shown in Table II, row 2, Bs is 0.68, dR/R=44.5% and lambda is1.1×10⁻⁶ which is equivalent to the reference device. Again, we hadimplemented NiFe(21%)10/NiFe(12%)₂₃ composite free layer and NiFeHf(15%)cap-MTJ into making 1 Mbit-MRAM chip. The switching field measured for a0.3×0.6 μm bit has been reduced to around 42 Oe. V₅₀ for the NiFeHf(15%)cap MTJ is still high around 750 mV in comparison to 660 my for thereference MTJ. dR/R measured at 300 mV bias is around 27% compared to21% for the reference MTJ. This amounts to a 29% improvement for MRAMdevice operation. Rp_cov (i.e. MTJ resistance uniformity) is around 1%,better than that of the Ru cap MTJ. One surprising result for usingNiFeHf to cap MTJ free layer was the very low “read” error-count (EC)for the 1-Mbit-chip. Furthermore, the NiFeHf(15%) cap MTJ showed a“write” margin in the full select/half select (FS/HS) test.

Analyses of the 1 Mbit-MRAM-chip indicate that for a 1.0×10⁻⁶ lambda,the MTJ would be better operated for a free layer with Bs around 0.60.It is desirable to have MTJ lambda as low as possible (e.g. 1.0×10⁻⁷) sothat a thicker free layer can be used (7) to form high performance MRAMdevices.

Using NiFeHf(15%) cap, MTJs with NiFe(17.5%)/NiFe(12%) composite freelayer were further made. The results are shown in Table III.

TABLE III Magnetic performance properties of the composite NiFe(17.5%NiFe(12%)free MTJ stack Free layer capping layer annealing Bs Hc RA,Ωμm² dR/R lambda NiFe(17.5%)8/NiFe(12%)19 NiFeHf(15%)25/Ta30/Ru100 250°C. 0.62 5.10 1099 47.1 1.00E−6 NiFe(17.5%)17/NiFe(12%)16NiFeHf(15%)25/Ta30/Ru100 ″ 0.68 4.85 1083 45.9 5.43E−7 NiFe(17.5%)38Ru30/Ta30/Ru100 ″ 0.68 4.55 769 37.1 −7.56E−7

Comparing Row 2 of Table III to Row2 of Table II, Bs (0.68) of the twoMTJs is the same but higher dR/R was obtained for theNiFe(17.5%)/NiFe(12%) MTJ. Magnetostriction measured for theNiFe(17.5%)/NiFe(12%)-MTJ is lower than that of NiFe(21%)/NiFe(12%)-MTJ.In addition to negative lambda NiFe(17.5%), the MTJ's of Table III wereannealed at 250° C. so that a lesser amount of Hf is being diffused intothe underlying NiFe free layer. For the 1-Mbit-MRAM chip wafers,NiFeHf(15%) cap-MTJ and Ru cap-MTJ with NiFe(17.5%) free layer (i.e.rows 2 and 3) both showed good FS/HS margins. Ru cap NiFe(17.5%)₃₈free-MTJ is the present POR for making the 1-Mbit MRAM chip.

One important characteristic pertinent to the NiFeHf cap-composite NiFefree-MTJ is that dR/R is not so much affected by the interface (i.e.NiFe(17.5%) in TABLE III) NiFe(x) alloy composition. For the referenceMTJ, as shown in row 3 of Table II and row 3 of Table III, however, dR/Rdecreases with decreasing Fe concentration.

Bs, dR/R and magnetostriction, of the NiFeHf cap MTJ can also be tunedby NiFeHf 15%) cap thickness. These results are shown in TABLE IV:

TABLE IV Bs, dR/R and magnetostriction as function of NiFeHf(15%)thickness NiFe FL cap RA dR/R Bs Hc He Hk lambda NiFe(21%)8/NiFe(12%)20NiFeHf(15%)-25A/Ta/Ru 1093 44.83 0.60 4.99 3.20 11.39 7.72E−7 ″NiFeHf(15%)-35A/Ta/Ru 1125 47.35 0.68 4.91 2.59 11.50 1.51E−6 ″NiFeHf(15%)-50A/Ta/Ru 1174 50.98 0.72 3.74 2.92 12.11 2.61E−6

As shown in TABLE IV, three MTJs having the same (composite) NiFe freelayer [i.e. NiFe(21%)8/NiFe(12%)22] were made with different NiFeHf(15%)cap thicknesses 25A, 35A and 50A. Bs for the three MTJs was,respectively, 0.60, 0.66 and 0.72. Based on the Bs data, it is notedthat the NiFeHf(15%) cap formed on top of NiFe(12%) free layer ismagnetic. For a 8″ diameter wafer, Bs per Angstrom for the NiFe(21%) is0.0185 nw/A, 0.0171 nw/A for NiFe(17.5%), and 0.0158 nw/A for NiFe(12%).It is calculated that Bs/A for the NiFeHf(15%) cap is around 0.0055webers per Angstrom. Thus, the NiFeHf(15%) layer of this invention notonly serves as the capping layer to getter oxygen in the underlying freelayer it also functions as a part of the free layer. Magnetostriction ofthis NiFeHf(15%) free layer has a slightly positive value. It is alsonoted that MTJs made with a thicker NiFeHf(15%) cap yield higher dR/R.

In terms of low magnetization materials, we had worked on making a MTJwith composite NiFe(12%)/NiFeHf(15%) free layer. In this invention thefree layer structure, NiFe(12%), is the interface magnetic layer and thetop NiFeHf(15%) layer also serves as a cap layer to getter oxygenoriginating in the underlying NiFe(12%) free layer which results informing a sharp AlOx/NiFe interface. Table V lists the magneticperformance properties of this low magnetization free layer MTJ.

TABLE V Magnetic performance properties of NiFe 12% free-MTJ Free layercapping layer annealing Bs Hc Hin RA dR/R lambda NiFe(12%)27NiFeHf(15%)40/Ta30/Ru100 250° C. 0.64 4.98 2.84 1035 45.1 −1.58E−7NiFe(12%)27 NiFeHf(15%)45/Ta30/Ru100 ″ 0.67 4.98 2.76 1035 47.9  1.70E−7 NiFe(12%)38 Ru30/Ta30/Ru100 280° C. 0.72 4.56 2.29 836 33.0−6.57E−6

As can be seen in row 2 of TABLE V, the MTJ free layer comprisingNiFe(12%)27 and NiFeHf(15%)45 has a total thickness equal to 72Angstroms. The average Bs/Angstrom is calculated to be 0.67/72=0.009nw/Angstrom. This amounts to a 53% of NiFe(17.5%), therein Bs/A is0.0171 nw/Angstrom. dR/R for this low magnetization/low magnetostrictionfree layer-MTJ is 47.1%, even higher than the NiFeHf(15%) cap MTJs madewith NiFe(21%) or NiFe(17.5%) interface layer.

In principle, dR/R is governed by the spin polarization of the interfacemagnetic layer such as NiFe(21%), NiFe(17.5%) or NiFe(12%) (presentinvention). In the case of a NiFe magnetic layer, spin polarization isincreased with increasing Fe content. Thus, NiFe having higher Fecontent is expected to have higher spin polarization, thereby yieldinghigher dR/R. From this new cap experiment, it turns out that a sharpAlOx/NiFe interface (resulting from cap gettering) is even more powerfulthan high polarization for maximizing dR/R.

1. A process for the manufacture of an MTJ device, that includes a freelayer having a magnetic moment, comprising: providing a substrate anddepositing thereon a magnetic pinning layer; depositing a magneticallypinned layer on said pinning layer; depositing a tunnel barrier layer onsaid pinned layer; depositing on said tunnel barrier layer a compositefree layer comprising first and second layers of NiFe whosemagnetostriction constants are of opposite sign, thereby canceling oneanother; depositing on said composite free layer a first cap layer ofNiFeHf; depositing on said first cap layer a second cap layer of Ru onTa; and then heating said device for a time and at a temperature thatare sufficient for oxygen trapped in said composite NiFe layer todiffuse into said NiFeHf layer, thereby sharpening an interface betweensaid tunnel barrier layer and said composite free layer.
 2. The processdescribed in claim 1 wherein said first NiFe layer comprises betweenabout 17.5 and 21 atomic percent Fe and said second NiFe layer comprisesabout 12 atomic percent Fe.
 3. The process described in claim 1 whereinsaid first NiFe layer is deposited to a thickness between about 5 and 20Angstroms and said second NiFe layer is deposited to a thickness betweenabout 10 and 30 Angstroms.
 4. The process described in claim 1 whereinsaid first NiFe layer comprises about 12 atomic percent Fe and saidsecond layer is NiFeHf(15%).
 5. The process described in claim 4 whereinsaid first NiFe layer is deposited to a thickness between 20 and 30Angstroms and said second NiFeHf layer is deposited to a thicknessbetween 25 and 50 Angstroms.
 6. The process described in claim 4 whereinsaid composite free layer has a magnetostriction value less than about2×10⁻⁶.
 7. The process described in claim 4 wherein said MTJ device hasa dR/R value of at least 40%.
 8. An MTJ device, that includes a freelayer having a magnetic moment comprising: a magnetic pinning layer on asubstrate; a magnetically pinned layer on said pinning layer; a tunnelbarrier layer on said pinned layer; on said tunnel barrier layer, acomposite free layer comprising first and second layers of NiFe whosemagnetostriction constants are of opposite sign; on said composite freelayer, a first cap layer of NiFeHf; and on said first cap layer, asecond cap layer of ruthenium on tantalum.
 9. The MTJ device describedin claim 8 wherein said NiFeHf layer contains about 15 or less atomicpercent of hafnium.
 10. The MTJ device described in claim 8 wherein saidtunnel barrier layer is selected from the group consisting of AlOx, MgO,AlHfOx, AlTiOx, and TiOx.
 11. The MTJ device described in claim 8wherein said first NiFe layer comprises about 17.5 atomic percent Fe andsaid second NiFe layer comprises about 12 or less atomic percent Fe. 12.The MTJ device described in claim 8 wherein said first NiFe layer has athickness between about 5 and 20 Angstroms and said second NiFe layerhas a thickness between about 10 and 30 Angstroms.
 13. The MTJ devicedescribed in claim 12 wherein said composite free layer has amagnetostriction constant that is less than about 2×10⁻⁶.
 14. The MTJdevice described in claim 12 wherein said MTJ device has a dR/R value ofat least 40%.
 15. An MTJ device whose free layer is a bilayer ofNi₈₈Fe₁₂, between 20 and 30 Angstroms thick, and (NiFe)₈₅Hf₁₅, about 45Angstroms thick, whereby said MTJ device has a dR/R ratio greater than47%.